1. Field of the Invention
This invention relates to superconducting cabled conductors and to a method for manufacturing them.
2. Background of the Invention
The possibility of using superconductors to obtain greater efficiency in electrical and magnetic applications has attracted considerable interest, particularly since the discovery of superconducting materials, such as the oxide superconductors, whose structures allow them to carry significant currents at relatively high temperatures, above about 20 Kelvin. However, to be practical outside the laboratory, most electrical and magnetic applications require flexible cabled lengths of conductor manufacturable with high packing factors at reasonable cost, in addition to high engineering current-carrying capacity.
High packing factor forms maximize performance per unit volume. Space constraints and the need to handle higher overall current densities are among the major design issues considered in most electrical applications.
Conductors which are flexibly cabled, that is, composed of twisted, helically wound, braided or otherwise transposed bundles of mechanically and electrically isolated conductor strands, are desired in many applications, including coils, rotating machinery and long length cables. In comparison to monolithic conductors of comparable composition and cross-section, cabled forms which are made from a number of conductor strands which are substantially mechanically isolated will have much higher flexibility. By substantially mechanically isolated is meant that the cable strands have some ability to move independently within the cable, although a degree of mechanical locking of the strands is usually desired for stability and robustness. Flexibility increases in proportion to the ratio between the cable cross-section and the strand cross-section.
In low temperature metallic superconducting conductors, cables which are made from a number of substantially electrically isolated and transposed conductor strands have been shown to have greatly reduced AC losses in comparison to monolithic conductors cf "Superconducting Magnets" by Martin Wilson (1983,1990), pp 197, 307-309, and it has been proposed that the same relation will hold for high temperature superconductors with more complex structures. AC losses are believed to decrease in relation to strand cross-section, cable cross-section and twist pitch. Litz cable, a cable with multiple electrically insulated strands assembled in a fully transposed configuration, is required for nearly all AC applications. For DC applications, multiple uninsulated strands may be cabled to obtain flexibility or mechanical robustness. The greater the number of strands in the cable, the more pronounced these advantages will be. Cabling is also desirable for ease in manufacturing, since cabling processes scale more easily than monolithic manufacturing processes.
However, most of the superconductors, such as superconducting ceramics of the oxide, sulfide, selenide, telluride, nitride, boron carbide or oxycarbonate types, which have shown promise for electrical and magnetic applications at relatively high temperatures are anisotropic superconducting compounds which require texturing in order to optimize their current-carrying capacity. It has not been considered feasible to form these into high packing factor, tightly transposed cable configurations because of the physical limitations of these materials. They typically have complex, brittle, granular structures which cannot by themselves be drawn into wires or similar forms using conventional metal-processing methods and do not possess the necessary mechanical properties to withstand cabling in continuous long lengths. Consequently, the more useful forms of high temperature superconducting conductors usually are composite structures in which the anisotropic superconducting compound is supported by a matrix material which adds mechanical robustness to the composite. For example, in preferred manufacturing processes for superconducting oxide composites, such as the well-known powder-in-tube (PIT) process or various coated conductor processes, the desired superconducting oxide is formed within or on a supporting matrix, typically a noble metal, by a combination of phase transformation and oxidation reactions which occur during the manufacturing process.
Even in composite forms, the geometries in which high-performance superconducting articles may be successfully fabricated from these materials are constrained by the necessity of "texturing" the superconducting ceramic to achieve adequate critical current density and by the electrical anisotropy characteristic of the superconductor. The current-carrying capacity of any composite containing one of these materials depends significantly on the degree of crystallographic alignment, known as "texturing" , and intergrain bonding of the superconductor grains , induced during the composite manufacturing operation. For example, the rare earth family of oxide superconductors, among the most promising and widely studied of the ceramic superconductors, require biaxial texture, a specific crystallographic alignment along two axes of each grain, to provide adequate current carrying performance Certain ceramic superconductors with micaceous crystal structures, such as the two-layer and three-layer phases of the bismuth-strontium-calcium-copper-oxide family of superconductors (Bi.sub.2 Sr.sub.2 Ca.sub.1 Cu.sub.2 O.sub.x, also known as BSCCO 2212, and Bi.sub.2 Sr.sub.2 Ca.sub.2 Cu.sub.3 O.sub.x, also known as BSCCO 2223), demonstrate high current-carrying capacity when uniaxially textured in the plane perpendicular to the current carrying direction. (Micaceous structures are characterized by highly anisotropic, plate-like grains with well-defined slip planes and cleavage systems.) In addition, many superconducting compounds may be partially textured by uniaxial texturing techniques. Those anisotropic superconducting compounds which are suitable for uniaxial texturing techniques have been considered especially promising for electrical applications because they can be textured by methods which are readily scalable to long length manufacturing.
In contrast to other known conductors, such as the normal and superconducting metals, the current carrying capacity of well-textured anisotropic superconducting composite articles will depend in large part on the relative orientations of their preferred direction, which is determined by the crystallographic alignment of their superconducting grains, and any current flow or external magnetic field. Because of their crystal structure, supercurrent flows preferentially in at least one of the directions lying within the plane normal to the c axis of each grain. Their critical current may be as much as an order of magnitude lower in their "bad" direction than in their "good" direction. Thus, an important consideration in fabricating high performance cables from these materials, which is not an issue in conventional cable fabrication, is finding a way to maximize the portions of the cable which do have the desired orientations. For optimum current-carrying capacity, it would be desirable to align all of the grains in the cable in parallel to one another along their relevant axes, e.g., at least the c axis for the uniaxial texturing typical of BSCCO 2212 or 2223, or at least the c axis and either the a axis or the b axis for the biaxial texture typical of the rare earth superconducting oxides, with each c-axis preferably perpendicular to the longitudinal axis of the cable regardless of the relative rotational orientations of the cable strands and filaments which contain them, but the twisting and bending characteristic required for conventional cabling are not readily adaptable to such uniform grain alignment.
Thus, an object of this invention is to provide a textured cabled conductor containing a textured anisotropic superconducting compound having substantial crystallographic grain alignment which is directionally independent of the rotational orientations of the strands and filaments in the cabled conductor, and a process for manufacturing such a cabled conductor.
Another object of the invention is to provide a novel cabled conductor manufacturing process that will allow strands of superconducting compounds which require texturing to be used with conventional high-speed cabling equipment.
Another object is to provide a mechanically stable, fully transposed high packing factor cabled conductor containing anisotropic superconducting compounds in a plurality of strands which may be highly aspected, and a method for manufacturing such a cabled conductor.
Another object of the invention is to provide a high packing factor, well-textured cabled conductor comprising a plurality of strands each comprising a micaceous or semi-micaceous superconducting oxide, most preferably BSCCO 2223 and a method for manufacturing such a cabled conductor.